This invention is concerned with an adjustable electric thermostat, particularly, but not exclusively, for electric home appliances.
For the adjustment of the reference temperature, such thermostats include a shaft having an outside knob, threadedly engaged with respect to the housing of the thermostat, so that the shaft end changes its position as the knob is rotated, thus displacing the home position of the heat probing member. In order for the threaded shaft to have a satisfactory engagement with the seat within which it rotates, the axial length of the seat should be, according to standard practice in mechanics, of at least about twice the shaft gauge, i.e., about 12 mm in the typical case of a 6 mm shaft. Seats having a smaller thickness make for unreliable couplings, because, with different dimensions due to manufacturing tolerances, either the resistance to rotation may be too high, or the coupling may be too loose and insufficient to assure that the angular position of the shaft be properly maintained in case of light shock or vibration.
In order to obtain the required thickness of the threaded portion, it is therefore usual to affix externally to the housing a metal washer of 10 to 12 mm thickness, usually attached to the metal sheet of the housing by riveting or punching. Such arrangement is expensive, and furthermore it undesirably increases the axial bulk of the thermostat. Because of this, it has been proposed to provide the threaded seat in a boss or lug obtained directly from the metal sheet from which the housing is made. The drawbacks caused by the small thickness (3 to 4 mm) of the seat are then avoided by cutting an axial slit in the hollow shaft and slightly flaring the slitted portion so that an elastic radial pressure is created between the shaft threads and the seat.
However, this approach, which requires an almost individual adjustment of the shaft, depending on the combinations of the dimensional tolerances which may arise between the seat and the shaft, can only be used when the thermostat is manually assembled, and it gives rise to unsurmountable difficulties in automatic assembly.